fix a bunch of dark-theme buttons

src/skins/vector/css/themes/_dark.scss

@@ -101,4 +101,15 @@ $lightbox-fg-color: #ffffff;
 $lightbox-border-color: #ffffff;
 
 // unused?
-$progressbar-color: #000;
+$progressbar-color: #000;
+
+// Nasty hacks to apply a filter to arbitrary monochrome artwork to make it
+// better match the theme.  Typically applied to dark grey 'off' buttons or
+// light grey 'on' buttons.
+.mx_filterFlipColor {
+    filter: invert();
+}
+
+.gm-scrollbar .thumb {
+    filter: invert();
+}